The present invention pertains to the switching power supply technical field. In particular, it pertains to a power supply suitable for portable computers.
Element 510 in FIG. 6 indicates an example of a switching regulator in the prior art.
This switching power supply 510 comprises a control circuit 520, an output transistor 511, an inductance element 513, an output capacitor 514 and a flywheel diode 517.
The output transistor 511 is comprised of an n-channel MOSFET, and the gate terminal is connected to the control circuit 520 and its operation is controlled by the control circuit 520.
The drain terminal of the output transistor 511 is connected to a high voltage power supply VP, and the source terminal is connected to one end of the inductance element 513. The other end of said inductance element 513 is connected to the output terminal 518. Between the output terminal 518 and the ground potential, the output capacitor 514 is connected, and the load 515 is connected in parallel with this output capacitor 514.
The anode terminal of the flywheel diode 517 is connected to the ground potential, and the cathode terminal is connected to the source terminal of the output transistor 511.
When the output transistor 511 conducts, the source terminal is connected to the high voltage power supply VP. In this state, the flywheel diode 517 obtains inverse bias, and current is supplied from the high voltage power supply VP to the output capacitor 514 and the load 515 through the inductance element 513.
When the output transistor 511 is cut off from that state, an electromotive force will be generated in the inductance element 513; the source terminal of the output transistor 511 will be swung to a negative potential; the fly-wheel diode 517 will obtain forward bias; and current will be supplied to the load 515 by means of the energy accumulated in the inductance element 513.
The aforementioned operation of the output transistor 511 is controlled by the control circuit 520. To explain the internal configuration of the control circuit 520, in said control circuit 520, the first and the second potential dividing resistors 521 and 522, a converter 525, a reference voltage circuit 526, a level shift circuit 533, a buffer circuit 535 and an auxiliary power supply circuit 539 are provided.
The voltage of the output terminal 518 is divided by the first and the second potential dividing resistors 521 and 522, and is input to the inverse input terminal of the comparator 525. The reference voltage output by the reference voltage circuit 526 is input to the non-inverse input terminal of the comparator 525, and the comparator 525 compares the divided voltage of the output terminal 518 and the reference voltage, and outputs the result of the comparison to the buffer circuit 535 through the level shift circuit 533.
The buffer circuit 535 operates by means of the auxiliary power supply circuit 539, and according to the result of the comparison, when the divided voltage of the output terminal 518 is smaller than the reference voltage, impresses a high voltage to the gate terminal of the output transistor 511 by means of the power supplied from the auxiliary power supply circuit 539, and makes the output transistor 511 conduct. When the situation is the opposite, it impresses a voltage of the same potential as the source terminal to the gate terminal, and cuts off the output transistor 511.
The aforementioned comparator 525 has a hysteresis characteristic, and controls such that when the output transistor 511 once conducts, the output transistor 511 will not be cut-off unless the divided voltage of the output terminal 518 decreases by the voltage of the hysteresis characteristic.
Because of the hysteresis characteristic, the load 515 is lighter, and when the output current is lowered, it is less likely that the voltage of the output terminal 518 will be lowered, thus, the oscillation frequency of the switching power supply 510 is lowered.
In general, when the switching power supply 510 is used for audio purposes of a computer, if the oscillation frequency of the switching power supply 510 exists in the voice band, there is a problem that the switching frequency will appear as noise in the speaker.
Therefore, with the aforementioned switching power supply 510, the oscillation frequency will be lowered in case of a light load, and when the frequency reaches an upper limit frequency FM FM≈20 kHz or lower, noise will occur.
Element L3 in the graph of FIG. 5 indicates a curve that illustrates the relationship between the magnitude of the load and the oscillation frequency of switching power supply 510. When the load 515 is lighter than the magnitude B, the oscillation frequency will be lower than the upper limit frequency FM of the voice band.
As a circuit whose oscillation frequency will be constant irrespective of the magnitude of the load, there is the switching power supply indicated with Element 610 in FIG. 7.
This switching power supply 610 comprises first and second transistors 611 and 612, the control circuit 620, the inductance element 613, and the output capacitor 614.
First and the second output transistors 611 and 612 are comprised of n-channel MOSFETs. The drain terminal of the first output transistor 611 is connected to the high voltage power supply VP, and the source terminal of the second output transistor 612 is connected to the ground potential.
The source terminal of the first output transistor 611 and the drain terminal of the second output transistor 612 are connected to each other. If the part where these are connected to each other is the node indicated with Element 619, one end of the inductance element 613 is connected to said node 619.
The other end of the inductance element 613 is connected to the output terminal 618, and between said output terminal 618 and the ground potential, the output capacitor 614 is connected.
The load 615 is connected in parallel with the output capacitor 614.
To the gate terminals of the first and the second output transistors 611 and 612, the control circuit 610 is connected, and the operation of the first and the second output transistors 611 and 612 is controlled by the control circuit 610.
Omitting the explanation of the parts that are the same as those in the switching power supply 510 explained above, the internal configuration of the control circuit 620 of this switching power supply 610 will be explained.
This control circuit 620 comprises first and second control circuits 630 and 640, which respectively control the operation of first and second output transistors 611 and 612.
The voltage of the output terminal 618 is divided by first and second potential dividing resistors 621 and 622; by means of the comparator 625, the divided voltage and a reference voltage output by the reference voltage supply 626 are compared, and the result of the comparison is output from the comparator 625.
If the divided voltage is higher than the reference voltage, a LOW signal will be output, and in the reverse case, a HIGH signal is output.
In first and second control circuits 630 and 640, first and second delay circuits 632 and 642 are respectively provided; and to the first delay circuit 632, the output signal of the comparator 625 is directly input, and to the second delay circuit 642, the output signal of the comparator 625 is input after being reversed by the inverter 641.
The output signal of the first delay circuit 632 is output to the first output transistor 611 through the level shift circuit 633 and the buffer circuit 635; and the output signal of the second delay circuit 642 is output to the second output transistor 612 through the buffer circuit 645.
First and second delay circuits 632 and 642 are configured so as to output after delaying only the timing at which the input signal changes from a LOW signal to a HIGH signal. As a result, with regard to first and second output transistors 611 and 612, of the timings at which the output signal of the comparator 625 switches, only a timing at which one of them turns from a cut-off state to a conductive state will be delayed.
First, in a state where the second output transistor 612 is cut off, when the first output transistor 611 conducts, and one end of the inductance element 613 is connected to the high voltage power supply VP, from the high voltage power supply VP, through the inductance element 613, current is supplied to the load 615 and the output capacitor 614.
Next, when the first output transistor 611 changes from conductive to cut-off, by means of the energy accumulated in the inductance element 613, current is supplied to the load 615 and the output capacitor 614. The current either flows through the parasitic diode in the second output transistor 612, or when the second output transistor 612 is conductive, flows in the opposite direction from normal from the source terminal toward the drain terminal.
In this case, if the second output transistor 612 is conductive, then the output capacitor 614 will start to discharge, and current will flow to the ground potential through the inductance element 613 and the second output transistor 612. Element 616 in FIG. 6 indicates the direction of the discharge current.
Because this discharge current consumes the charge of the output capacitor 614, the voltage of the output terminal 618 will promptly be lowered even in case of a light load.
Then, when the divided voltage of the output terminal 618 becomes lower than the reference voltage, the second output transistor 612 will be cut off without any delay, and then, the first output transistor 611 will become conductive after the delay time set by the first delay circuit 632 has passed.
At the point when the first output transistor 611 has become conductive, due to the discharge of the output capacitor 614, even with the light load, the voltage of the output terminal 618 has been lowered to the same level as in the case of a heavy load.
Therefore, with regard to this switching power supply 620, the oscillation frequencies of first and second output transistors 611 and 612 will be approximately constant irrespective of the magnitude of the load.
Element L2 in the graph of FIG. 5 is a graph that illustrates the relationship between the magnitude of the load 615 and the oscillation frequency of the switching power supply 610.
With regard to this switching power supply 610, the higher the oscillation frequency, the smaller the output ripple voltage, and the easier it is to maintain the voltage of the output terminal 618 constant. Therefore, so as not to lower the voltage of the output terminal 618 with a heavy load, the oscillation frequency is set significantly higher than the upper limit frequency FM of the voice band. Therefore, on the other hand, with a light load, the oscillation frequency is too high, and the first and the second transistors 611 and 612 switch unnecessarily; thus the efficiency of the light load will be lowered, and therefore, the above is not suitable for a portable computer.
The present invention was created to solve the problems of the aforementioned prior art. The object is to offer a power supply that is suitable for portable computers.
To solve the aforementioned problem, the driving signal supply circuit of the present invention is a driving signal supply circuit, which supplies driving signals to the first and the second switching transistors of a switching regulator including the first switching transistor connected between a power supply voltage supply terminal and a first node; the second switching transistor connected between the aforementioned first node and a reference voltage supply terminal, which can be in a conductive state when the aforementioned first switching transistor is in a cut-off state; a coil with one end connected to the aforementioned first node; and a smoothing capacitor connected between the other end of the aforementioned coil and the reference voltage supply terminal, and comprises a comparing circuit, which compares a first detected voltage corresponding to the output voltage of the switching regulator with a first reference voltage and outputs a first comparison signal; a first driving circuit, which inputs the aforementioned first comparison signal and outputs a first driving signal to drive the aforementioned first switching transistor; a second driving circuit, which inputs the aforementioned first comparison signal and outputs a second driving signal to drive the aforementioned second switching transistor; a second comparing circuit, which compares a second detected voltage corresponding to the voltage of the aforementioned first node with a second reference voltage and outputs a second comparison signal; and a logic circuit, which inputs the aforementioned second comparison signal and outputs an inhibiting signal to inhibit conduction of the aforementioned second switching transistor.
Further, in the driving signal supply circuit of the present invention, preferably, the aforementioned second reference voltage changes corresponding to the power supply voltage.
Furthermore, in the driving signal supply circuit of the present invention, the aforementioned first driving circuit comprises a first delay circuit that provides a first delay time to the rise or the fall of the aforementioned first comparison signal; and the aforementioned second driving circuit comprises a second delay circuit that provides a second delay time to the rise or the fall of the inverse signal of the aforementioned first comparison signal, and a logic means that does a predetermined logical calculation of the output signal of the aforementioned second delay circuit and the aforementioned inhibiting signal and outputs a logic signal.
The driving signal of the present invention is configured as described above, and a discharge period is provided when the second switching transistor discharges the smoothing capacitor. As the voltage of the smoothing capacitor is lowered corresponding to the amount of electric charge discharged during this discharge period in case of a light load, the output voltage will be lowered. Therefore, the period in which the first switching transistor is cut off will be shorter, and the lowering of the oscillation frequency of the switching regulator in case of a light load will be controlled.
Then, in the driving signal supply circuit of the present invention, the second switching transistor is forced into a cut-off state by the inhibiting signal output corresponding to the voltage of the first node, and the aforementioned discharge period is ended. In this manner, as the aforementioned discharge period is forced to end, and excessive discharge of the smoothing capacitor is prevented, the lowering of the efficiency of the switching regulator is controlled.
In this manner, the switching regulator of the present invention can prevent lowering of the oscillation frequency into the voice band in case of a light load, and can maintain power conversion efficiency.